FIG. 1 of the accompanying drawings illustrates a wireless telecommunications network 10, which support communication with terminals in a number of cells (A, B, C, D) each of which is served by a radio base station 2. Each communication cell covers a geographical area, and by combining a number of cells a wide area can be covered. A mobile terminal 4 is illustrated communicating in cell A, and is able to move around the network 10.
A base station 2 contains a number of receivers and transmitters to give radio coverage for one or more cells. Each base station 2 is connected to a network “backbone”, or core network infrastructure (not shown), which enables communications between base stations and other networks. The example system of FIG. 1 shows one base station per cell.
An important concept in such a network is the cell and its neighbors. During a call a mobile terminal 4 typically is moved in the geography, and when so doing leaves a first cell and enters a new cell that neighbors the first cell. The cell may be changed several times and the process of changing the cell that supports a radio link with the terminal is called handover. A list of the known neighbors, the so called “neighbor cell set”, is important both for the network 10 and for the mobile terminal 4 to enable reliable handover between cells. The network 10 can store information relating to a set of neighbor cells for each cell in the system. Evaluation of the best cell for supporting a radio link with the mobile terminal is based upon measurements made by the terminal on the serving cell and on other cells than the serving cell. The neighbor cell list is needed for mapping measurements and handover decisions to a target cell identity and possibly applying specific parameters or rules for the target cell. It will be readily appreciated that the cell boundaries are not sharply defined, but will in practice be somewhat blurred as the range of the base stations will overlap with one another.
In existing systems, the mobile terminal 4 detects and measures cell operating parameters for neighboring cells by measuring on their broadcast channels. One measured operating parameter is a cell non-unique identifier which typically consists of a physical layer identifier such as a scramble code which is non-uniquely assigned to the cell. Operating parameters also relates to the signal quality of the neighbor cell such as signal strength, signal quality and timing information. When the quality of a neighbor cell is considered better than the current serving cell, a handover from the serving cell to the chosen neighbor cell is executed by the network. The neighbor cell then becomes the serving cell for the mobile terminal.
Typically in a WCDMA (wideband code division multiple access) system, the mobile terminal detects Common Pilot Channel (CPICH) transmissions from surrounding cells, in order to determine id (scramble code) and timing information.
When the mobile reports the neighbor cell signal quality measurements to the network, the cells' respective identities become important. Typically, cell identities are reused for more than one cell. The reuse of identities means that cells may be confused with one other, since the serving cell may have neighbor cells sharing the same identity information.
In the cells are also broadcasted unique cell identities. The unique cell identity is carried on the network layer. Its main use is for special purpose terminals that are used by an operator of the system for testing and tracing of problems in the network. The unique cell identity is not repeated as often as the non-unique cell identity and is more complicated for a terminal to detect. This is in contrast to the physical layer which carries physical data needed for supporting the radio link.
Since the cells' physical layer identifiers are non-unique, populating and maintaining the neighbor cell sets can never be fully automatic. Human efforts are needed to resolve conflicts where the serving cell has multiple neighbors using the same non-unique identifier. A further problem is handover failures owing to the candidate cell having been incorrectly identified.